Carbon nanotubes (CNTs) can be viewed as a sheet of Carbon that has been rolled into the shape of a tube (end capped or non-end capped). CNTs having certain properties (e.g., a “conductive” CNT having electronic properties akin to a metal) may be appropriate for certain applications while CNTs having certain other properties (e.g., a “semiconducting” CNT having electronic properties akin to a semiconductor) may be appropriate for certain other applications. CNT properties tend to be a function of the CNT's “chirality” and diameter. The chirality of a CNT characterizes its arrangement of carbon atoms (e.g., arm chair, zigzag, helical/chiral). The diameter of a CNT is the span across a cross section of the tube.
Because the properties of a CNT can be a function of the CNT's chirality and diameter, the suitably of a particular CNT for a particular application is apt to depend on the chirality and diameter of the CNT. Unfortunately, current CNT manufacturing processes are only capable of manufacturing batches of CNTs whose tube diameters and chiralities are widely varied. The problem therefore arises of not being able to collect CNTs (e.g., for a particular application) whose diameter and chiralities reside only within a narrow range (or ranges of) those that have been manufactured.
United States Patent Application Publication U.S. 2004/0120880 by Zhang, Hannah and Woo (hereinafter “Zhang et al.”) and entitled “Sorting of Single-Walled Carbon Nanotubes Using Optical Dipole Traps” teaches that CNTs of specific chirality and diameter will posses electrical dipole moments that will cause the CNT to exhibit characteristic “attraction/repulsion” behavior under an applied time-varying electric field. As such, Zhang et al further teaches a technique that uses the characteristic “attraction/repulsion” behavior as a basis for collecting “targeted” CNTs of specific tube chirality and diameter.
With respect to a CNT's “attraction/repulsion” behavior, Zhang et al. teaches that the system energy of a CNT placed in a time-varying electric field is U=−½ε0χE2 where ε0 is the permitivity of free space, χ is the dielectric susceptibility of the CNT and E2 is the intensity of the time-varying electric field. The dielectric susceptibility χ describes the collective orientation and strength of the individual electric dipole moments of the CNT in response to the applied time-varying electric field. According to Zhang et al., the dielectric susceptibility χ is a function of the frequency of the applied electric field; and, more importantly, that the collective “direction” of the CNT's electric dipole moments change as a function of frequency.
Specifically, for applied electric field frequencies beneath a “resonant” frequency, the dipole moments collectively “point” in a direction that causes the CNT to move towards increasing electric field intensity (i.e., the CNT is attracted to regions of increasing electric field intensity because lower system energy results from higher electric field intensities); while, for applied electric field frequencies above the aforementioned resonant frequency, the dipole moments collectively “point” in a direction that causes the CNT to move away from increasing electric field intensity (i.e., the CNT is repelled from regions of increasing electric field intensity because higher system energy results from higher electric field intensities). If the frequency of the applied time-varying electric field is at the resonant frequency, the collective pointing direction and motion of the CNT is unstable.
Zhang et al also teaches that the specific resonant frequencies of a CNT are a function of its energy bandgaps, and that, the energy bandgaps of a CNT are a function of the CNT's chirality and diameter. Hence, the aforementioned characteristic attraction/repulsion behavior of a CNT in response to an applied time-varying electric field is a function of the CNT's chirality and diameter.
Zhang et al. further describes a technique for sorting CNTs based upon the above described attraction/repulsion behavior. In particular, if an electric field is applied to a group of CNTs having diverse chiralities and diameters (e.g., such as a batch of CNTs produced by a single manufacturing process run), a specific CNT can be collected through the application of a time-varying electric field whose frequency is tailored in light of the resonant frequency of the CNT sought to be collected. FIGS. 1a through 1c demonstrate the technique in more detail.
FIG. 1a shows a fluidic flow 103 containing manufactured CNTs. It is assumed that the manufactured CNTs have various combinations of diameter and chirality. For simplicity, FIG. 1a shows only two types of manufactured CNTs: 1) a first group 105, 107, 110, 111, 112, 114, 117, 119 having a first chirality and diameter combination; and, 2) a second group 106, 108, 109 113, 115, 116, 118, 120 having a second chirality and diameter combination. All of the CNTs 105 through 120 enter the apparatus as part of fluidic flow 1031. A second fluidic flow 104 flows along side fluidic flow 103.
The general idea is that a particular type of CNT, such as the CNTs associated with the first group defined above, is to be extracted from fluidic flow 103 and introduced to fluidic flow 104. Thus, CNTs of the first type will flow out of the apparatus as part of fluid flow 1042 and CNTs of the second type will flow out of the apparatus as part of fluid flow 1032.
The extraction process uses the electric field component of a laser beam to apply the time-varying electric field. A laser beam spot 101 is drawn as being impingent upon fluid flow 103. The laser beam is focused and thus converges to a source image 102 further along the x axis approximately within the center of fluid flow 103's cross section (FIG. 2, which is discussed in more detail ahead, provides a three dimensional perspective of a laser beam focused as just described).
A focused point 102 in the center of the fluid flow causes the electric field intensity of any region that is illuminated by the laser beam to increase in the direction toward the focused point 102. Therefore, by selecting a laser beam frequency that is beneath the resonant frequency of the first group of CNTs but above the resonant frequency of the second group of CNTs, CNTs from the first group will be attracted toward the focused point 102 while CNTs from the second group will be repelled from the focused point 102.
At the instant of time represented by FIG. 1a, sweeping the laser beam from fluid flow 103 to fluid flow 104 will cause CNTs 105 and 107 to be pulled, as a consequence of their attraction to focused point 102, into fluid flow 104; while, CNT 106, as a consequence of its repulsion from point 102, will remain in fluid flow 103. The situation after the sweeping of the laser beam is depicted in FIG. 1b. 
It is clear from the situation of FIG. 1b that CNTs 105 and 107 will exit as part of exit flow 1042 and that CNT 106 will exit as part of exit flow 1032. FIG. 1c shows the situation if the laser beam is swept again from flow 103 to 104 so as to capture CNTs 110, 111 and 112 from flow 103 and introduce them to flow 104. It is also clear that repeating this sweeping motion will cause the CNTs of the first group to exit as part of exit flow 1042 and that CNTs of the second group will exit as part of exit flow 1032. Thus, the sorting of CNTs is accomplished.